1 . Field of the Invention
The invention relates to a lubrication structure of a differential gear unit. More particularly, the invention relates to a lubrication structure of a differential gear unit that lubricates a bearing that rotatably supports a differential case that houses a differential mechanism therein by a hollow supporting portion of a case of a power transmitting apparatus.
2 . Description of the Related Art
A differential gear unit incorporated in a transaxle of a vehicle such as an automobile includes a housing portion and a differential case that is rotatably housed in the housing portion. A drive pinion that is driven by a propeller shaft or the like is rotatably housed in the housing portion, and a ring gear that is in mesh with this drive pinion is mounted to the differential case by a plurality of bolts.
This differential case is rotatably supported via a tapered roller bearing in a case of a power transmitting apparatus. This tapered roller bearing receives a load not only in the radial direction but also in the thrust (i.e., axial) direction, so it is critical that it be kept well lubricated.
Normally, oil for lubricating the drive pinion and the ring gear is stored in the housing portion. In order to lubricate the tapered roller bearing with this oil, the oil level is raised so that the tapered roller bearing is immersed in oil (see Japanese Patent Application Publication No. 5-106710 (JP-A-5-106710), for example).
Meanwhile, the bolts used to fasten the ring gear to the differential case are susceptible to fluctuating loads and vibration, so folded washers or the like are used. In particular, the bolts that fasten the ring gear to the differential case are attached in a circle centered around the rotational axis of the differential case and the ring gear. Accordingly, the amount of slippage that occurs between the heads of the bolts and the seating surface becomes greater farther radially outward. If the slippage that occurs between the heads of these bolts and the seating surface is in the same direction as the rotational direction in which the bolts loosen, the clamping force of the bolts may gradually decrease.
One known example of technology for maintaining the clamping force of bolts includes hexagon bolts, a flange portion of a differential case through which shafts of the hexagon bolts pass and with which heads of the hexagon bolts engage, and a ring gear that is fastened to the flange portion of the differential case by the hexagon bolts. Recessed portions that recess the heads of the hexagon bolts are formed in the flange portion of the differential case, and resin is filled between the recessed portions and the heads of the hexagon bolts that are recessed in the recessed portions. This resin is able to inhibit the clamping force of the hexagon bolts from gradually decreasing (see Japanese Patent Application Publication No. 2009-480289 (JP-A-2009-180289), for example).
However, with the lubrication structure of a differential gear unit described in JP-A-5-106710 , in order to lubricate the tapered roller bearing with the oil filled in the housing portion, the oil level must be raised to immerse the tapered roller bearing in oil. Thus, the amount of oil in the housing portion ends up increasing, which in turn increases the rotational resistance of the differential gear unit as well as reduces the power transmission efficiency.
Also, with the technology described in JP-A-2009-180289 , the recessed portions that recess the heads of the hexagon bolts are formed in the flange portion of the differential case, and resin is filled between these recessed portions and the heads of the hexagon bolts that are recessed in these recessed portions. As a result, the manufacturing cost of the differential gear unit ends up increasing. In particular, a recessed portion and filling resin must be provided for each hexagon bolt, so the manufacturing cost of the differential gear unit ends up increasing as the number of hexagon bolts increases.